Proposed Definitions for XMM Operations

In order to define nomenclature for operational terminology regarding the use of the XMM instruments, the following definitions have been compiled. This reflects the fact that the operations of the ground segment are being specified, and we must have nomenclature that adequately describes the content and format of data sets. This can be at variance with terminology commonly employed in the description of detector operation.

It is recommended that as these will be followed in all official Project documents, that the instrument teams conform as closely as possible with these definitions in their own documentation and activities.

See also the glossary of commonly used terms.

Description of typical Observations

Starting from the first level of operation, we assume that before an observation starts, the spacecraft will slew to a commanded direction. Subject to small trim manouvers (assuming deliberate rastering is not allowed except by explicit pointing change requests or multiple observation requests TBC) this can be considered a ``fixed'' pointing direction to the celestial sphere. One might conceive that an observer could be proposing to look at a cluster of galaxies, a large SNR or stellar cluster, and in order to ensure complete coverage of the system, might ask for some coarse re-pointing manouvers over the course of his/her observation. Thus it is possible for one proposal to request a number of pointings.

At any one pointing, the observer might require different aspects of the source to be investigated, which cannot be handled by a single instrument configuration. Suppose, for example, that complementary data are required to be taken in different time resolutions, different filters, different windows etc.. For such an eventuality, data will be collected for a specified time, and the resulting data set collected on the ground might be termed an "exposure" - this is especially relevant for the OM, where data are actually collected on-board for one image made in a configuration, but complicated by the fact that the OM might simultaneously generate a parallel stream of higher time resolution data. The X-ray instruments will not produce images/spectra until after some ground-based processing, but the concept of an exposure which comprises data from a block of time which will be treated in the same way is a useful terminology, and if it can be agreed observatory-wide will ease the understanding for operators and users alike. In this sense, it is proposed that one exposure is linked to a single file within an ODF (Observation Data File). The ODF is produced for each pointing, and therefore its structure must contain pointers or a directory to refer to individual exposures.

This does however raise the question of shorter periods of time associated with (for example) discrete readout periods of the detectors. For the X-ray instruments' CCDs, It is proposed to use the term "frame" describing the reading out of a complete or partial set of CCD pixels.

All instruments seem to have the facility/requirement to be able to maximise count rate capability, by reading out only limited regions of one or more detectors. This spatial censoring can first be performed at the CCD readout level, but in principle also in the data handling units. It is proposed to call this restricted area a "window", whether it is defined in hardware or by software in the processing.

There can be some confusion about the words ``sources'' and ``targets''. It is proposed that TARGET refers to the actual object or co-ordinates within an extended object that the instruments are commanded to acquire. A SOURCE might be any other object detectable within the fields of view.

Summary Definitions

OBSERVATION

the set of operations requested by an observer to fulfill the science investigations into a source, described in the proposal. This may comprise more than one Observatory/instrument configuration

POINTING

a period of time during which the reference axis is held to a specific commanded location in the celestial sphere

MODE

the instrument set-up which can be called by telecommand and with which are associated some user specified parameters. A unique data format may be associated with each mode. A specific set-up within the ground segment is required to process/display instrument data from each mode

CONFIGURATION

for each instrument, and/or the Observatory, a fixed set of commandable conditions which are set up in order to take science data. Because filters or window sizes may be changed within one mode by varying user definable parameters, any mode may have a number of possible configurations. In addition, it is possible for an instrument configuration to employ two modes simultaneously.

EXPOSURE

the period of time during which data is generated by an instrument when held in a fixed configuration. By implication all data from one exposure will be in the same format, probably in a single identifiable data file which is defined and/or pointed to from the ODF directory

For the X-ray instruments, then an exposure comprises many CCD frames which are assembled on ground. For the OM an exposure is accumulated in memory before telemetering. More than one subsequent exposures can be made and telemetered under the same instrument configuration of the OM.

FRAME

a data set generated by a detector during a quantised period, such as a single CCD readout or an interim photon-counting image accumulated in a memory. For the X-ray cameras the frames will be identified by a changing time stamp associated with a CCD readout. OM has BLUE CCD frames which are not usually transmitted as individual data units. OM has BLUE tracking FRAMES, which are accumulated in memory. Again these are not telemetered as individual data units.

The OM defines a separate temporal unit in timing mode as a TIME SLICE, in order to reduce confusion in nomencalture

FRAME TIME

the time for collection of photons within one CCD or other (eg. tracking) FRAME.

C. Erd, D. Lumb, R. Much
November 17, 1995